1. Field of the Invention
The present invention relates to semiconductor manufacturing, monitoring of vacuum chamber cleanup and trace gas analysis.
2. Description of Related Art
The semiconductor manufacturing industry achieves higher and higher levels of integration of integrated circuits dealing with contamination and yield problems. Frequent pump/purge cycles with high purity purge gas in between reactive process gas cycles are hereto applied. These pump/purge cycles clean out process gas from a prior reactive process cycle, as well as species like oxygen and moisture that might have entered the tool's chamber. In-situ, real time process monitors of all kind of process variables are considered critical for current and future developments. An example of a new type of monitor is the measurement of trace concentrations of moisture in the exhaust of a semiconductor-manufacturing tool by diode laser based absorption spectroscopy.
Traditionally, monitoring the pressure during pumpdown can reveal gross problems with leaks or excessive outgassing. A pressure gauge, however, is not specific for a particular species. Moreover, most information is in the very tail part of a drydown curve and, in a normal operating procedure, there is simply no time to wait that long. The purge and pump cycles are often done simultaneously, opening wide to the pump while entering a stream of purge gas with a total cycle time in the order of 10 or 20 seconds.
Species specific monitoring can be done with a mass spectrometer that is then called a residual gas analyzer (RGA). Such RGA operates at 10−5–10−6 Torr requiring differential pumping when probing a purge gas at a higher pressure. Mass spectrometers have a residual spectrum at the lower masses that prevents measuring water and oxygen at levels below a part per million.
Some of the above and other in-situ measurement techniques for pump/purge cycles can detect the removal of previously introduced reactive process gas and rare intrusions of excessively high contamination levels. Even to accomplish this limited objective, in-situ techniques, such as RGA, are expensive, maintenance intensive and require elaborate calibration. Moreover, none are sensitive enough and fast enough to follow the actual progress of mentioned 10–20 seconds pump/purge cycles at partial pressures of relevant contaminants in the order of 10–12 atmosphere.
Optical (none afterglow) emission spectroscopy finds practical applications in the semiconductor manufacturing industry. The observation of the spectrum emitted by the process plasma during a process and its evolution over time provides information about the completion of certain reactions such as in end point detection. The presence of a broad background spectrum, however, obscures any weak emission from trace species.
Non-in-situ qualification of cylinder gas or bulk supply of electronic grade nitrogen and argon by metastable transfer emission spectroscopy (MTES) is investigated for oxygen-containing impurities by J. W. Mitchell et al., Analytical Chemistry 1986, 58, 371–374, which is incorporated by reference herein in its entirety. The required bulky and maintenance intensive vacuum technology in such a flowing afterglow application of MTES prevents it from being applied in any practical gas analytical instrumentation.
Non-in-situ gas analysis at parts per billion levels is routinely performed on line for the purge gas prior to its entering into the process tool. At that point, the purge gas is at higher than atmospheric pressure while the analysis can be performed in the order of minutes. The very sensitive and contaminant specific instruments for such purge gas analysis are used downstream of the large purifiers that provide gas to a semiconductor plant's gas distribution manifold. Such instruments are still too complex and expensive to be used as simple end of life detection for point of use purifiers that are used at the purge gas inlet of an individual tool.
Such non-in-situ analysis prior to entering the tool can sometimes also be performed for the reactive process gases although, highly desirable hygrometry of corrosive gases can only be done economically per cylinder, off line, in a laboratory environment.
Even non-in-situ applications requiring only ppm level detection limits can be troubled by the lack of speed, reliability, robustness and ease of maintenance of current state-of-the-art instrumentation. Such applications include a 10-percent increased efficiency of the recovery of argon from an air separation system described in U.S. Pat. No. 4,784,677 entitled “Process and Apparatus for Controlling Argon Column Feedstreams” by Al-Chalabi, and U.S. Pat. No. 5,448,893 entitled “Process for Maximizing the Recovery of Argon From an Air Separation System at High Argon Recovery Rates” by Howard et al., both of which are incorporated by reference herein in their entireties, requiring ideally an analyzer with a one-second response time. U.S. Pat. No. 4,801,209 entitled “Process and Apparatus for Analyzing a Gaseous Mixture and a Visible Emission Spectrum Generator Therefor” by Wadlow, which is incorporated by reference herein in its entirety, describes an analyzer for the maximization of argon recovery, but lacks the robustness and reliability needed for application in a process environment.